I have a 1995 E320 with no body rust anywhere. I just noticed this problem. If I park outside in the rain, when I open the trunk, rainwater pours out of the trunk lid where the locking mechanism is. Water seems to pool behind the license plate, then when the lid is vertical, all this water pours onto your head while you're looking in the trunk.

I cannot for the life of me figure out where the water is getting in and how it is collecting inside the trunk lid by the license plate. Has anyone else encountered this???

I placed paper towels around the trunk, closed the trunk lid, then sprayed all around with the garden hose. Then opened the trunk and saw that water was entering from around the rubber trunk seal.

Also you can stick a piece of paper at various points of the rubber seal, close the trunk lid, then see if the paper will slide (gap between rubber and lid) or is firmly gripped by the rubber seal (paper will not slide).

I found a replacement rubber seal at Amazon.com. Also used 3M weatherstrip adhesive.

That solved most of the leaking. There remained a bit more leaking which I traced to a rear light assembly which was not attached properly - missing nuts - and part of the gasket was twisted so it was not in place like it should be.

For that I just totally removed the tail light assembly, placed the gasket on properly, bought some more nuts, then fastened on properly.

it also might be coming from the moulding that holds the license plate lights. I check mine yesterday on my '95 E300 and it appears to have a gasket there? look for water stains on the liner under the truck lid too